Time of flight (TOF) sensors typically utilize charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based technologies that are able to sample at high speed. The typical application is for point distance sensing or three-dimensional (3D) imaging in which the scene of interest is actively illuminated with modulated illuminating radiation and the sensor sampling is performed synchronously with the illuminating radiation. High speed sensors are also useful in other applications such as fluorescence lifetime imaging.
Generally, in these sensors light is converted into electrical charge carriers, usually electrons, but holes could also be used, in a photosensitive region. Drift fields are created in this region to transfer the charge carriers to one or more integration gates where the charge is stored until a readout stage. In the typical example, specific integration gates are assigned to different phases within the period of the stimulation radiation and the drift fields are modulated synchronously with the simulation radiation in order to move the charge carriers from the photosensitive region to the integration gates for the phase assigned to that gate.
An early example of a TOF sensor was disclosed in the German patent DE4440613C1 (Spirig, “Vorrichtung und Verfahren zur Detektion eines intensitätsmodulierten Strahlungsfeldes”, 1996). See also U.S. Pat. No. 5,856,667. A demodulation device is presented that samples the impinging optical sinusoidally-modulated light signal n times. Charge coupled devices are used for the detection of the light signal and the subsequent transport of the photo-generated charges.
Later, German patent application DE19821974A1 (Schwarte, Vorrichtung und Verfahren zur Erfassung von Phase und Amplitude elektromagnetischer Wellen, 1999), see also U.S. Pat. No. 6,825,455 B1, disclosed a photon-mixing element for a TOF sensor. In order to get a pixel with high-sensitivity and high-speed demodulation facility, a combined structure of stripe-like elements, each of them with short transport paths, is proposed. Nevertheless, the stripe-like structure leads to a poor fill-factor because the regions between the stripes are not photo-sensitive.
Another approach for large-area demodulation pixel for a TOF sensor with high sensitivity and high demodulation speed is given in the English patent application GB2389960A (Seitz, “Four-tap demodulation pixel”, 2003). See also US. Pat. Publ. No. US 2006/0108611 A1. A high-resistive photo-gate of rectangular shape and large size generates a drift-field within the semiconductor substrate enforcing the photo-generated charges to drift to the particular sampling node. Here, any delay of the sampling signal arising on the photo-gate due to large resistance-capacitance (RC) times can reduce the performance of such demodulation pixels. In particular, high frequencies are difficult to realize when many pixels are controlled at the same time. Then the external electronics and their limited driving capability of large capacitances represent the constraining factor.
All pixel structures mentioned above have a common property that the lateral conduction of the photogenerated charges into a specific direction is always related to the push-pull signal on a gate structure spanning the photosensitive photodetection area of the pixel. In order to get higher sensitivities, the photodetection area has to be enlarged, this results in either increased parasitic capacitances that have to be switched or longer transport paths. Both aspects are undesirable because they detrimentally impact the speed of these devices. If the switching gate capacitances increase, the speed limitations are dictated by the driving electronic components. On the other hand, long transport paths increase the time required for the photo-generated charges to reach storage.
WO 2007/045108 A1 presents a newer TOF sensor example. Here, the drift field over most or all of the photosensitive area is basically static in time. The static or quasi static field in the photosensitive region moves or the dumps the charge carriers into a typically smaller modulated region, which may or may not be photosensitive. The charge carriers are then swept from the modulated region into integration regions or gates synchronously with the modulated signal. This newer system can operate at much higher frequencies because modulation is over a much smaller area, having a lower intrinsic capacitance, whereas transport within the large photosensitive region can be optimized for speed. The newer demodulation device avoids trade-offs between the sensitivity/fill-factor and the demodulation speed. Both aspects can be optimized in the pixel at the same time.
U.S. Pat. No. 6,483,094B1 presents another technology to measure the time of flight. The invention consists of a solid state optical shutter device to demodulate the received light and therefore, in combination with a conventional imager, is able to resolve the TOF information. The optical shutter is placed in front of the imager. It consists of a photodiode as input and a light emitting diode as output. The gate in the photodiode and the light emitting diode can be controlled to enable/disable the electron transfer from the photodiode to the LED. This allows a high-speed demodulation of the received modulated light, hence gathering TOF information.
In all of these designs, synchronization is critical between the modulated illuminating radiation and the sensor sampling or the optical shutter demodulation respectively. Measuring distances based on the TOF require extremely high timing stability for all components: modulated light source, image sensor, and the control circuitry. Temperature changes and component aging, however, result in small timing shifts in operation of the modulated light source, image sensor and driving control circuitry.